This disclosure is directed to methods of manufacturing phenolphthalein compounds, in particular methods using a heteropolyacid catalyst.
Phenolphthalein compounds are useful as a starting material to make a wide range of products. For example, phenolphthalein compounds are important raw materials for the synthesis of 3,3-bis(4-hydroxyphenyl)phthalimidine and 2-hydrocarbyl-3,3-bis(4-hydroxyaryl)phthalimidines, in particular 2-phenyl-3,3-bis(4-hydroxyphenyl)phthalimidine (PPPBP). These phthalimidine compounds can be used in the manufacture of homopolycarbonates and copolycarbonates.
Phenolphthalein has been produced in a homogenous process in which phthalic anhydride is reacted with phenol in the presence of zinc chloride (ZnCl2) as a catalyst. In U.S. Pat. No. 2,522,939, Gamrath disclosed improving this widely used process by the addition of chlorosulphonic acid as an activating agent (also referred to as a promoter) for the zinc chloride. This process has become commercially used because of the high purity, selectivity, and yield obtained.
Presently available manufacturing processes for phenolphthalein are time consuming and require large amounts of energy and chemicals, as well as complex equipment. U.S. patent application Ser. No. 11/626,671 discloses an improved method for producing and purifying phenolphthalein compounds on a commercial scale that requires fewer resources. In particular, after reacting a phthalic anhydride compound and a phenol compound in the presence of a catalyst and a promoter to form a reaction mixture comprising the phenolphthalein compound, the reaction mixture is treated with a solvent system to form a slurry. The slurry can then be filtered to obtain a solid material, which, after washing in water at an elevated temperature, comprises the phenolphthalein compounds in high purity. This commercial process, however, still employs zinc chloride and chlorosulphonic acid to catalyze the reaction.
There are several significant challenges associated with processes for manufacturing phenolphthalein using zinc chloride. In commercial practice, the zinc chloride is used in relatively large amounts, 0.6 mole of zinc chloride per mole of phthalic anhydride. The used catalyst must be separated from the reaction mixture after quenching of the reaction. The difficulty of separating the catalyst, which is used in slurry form, is increased because the slurry becomes very viscous over time. Importantly, the large amounts of used catalyst cannot be reused and must be disposed of safely.
Different approaches to making phenolphthalein have been employed both on a laboratory and commercial scale. For example, ion exchange resins are also known for phenolphthalein preparation, as disclosed in U.S. Pat. No. 4,252,725. However, commercial manufacture is very demanding in terms of yield, purity, and other characteristics. The use of zinc chloride in the preparation of phenolphthalein, despite its problems, remains the industry standard in the absence of a practical replacement.
It would be desirable to develop a process for the preparation of phenolphthalein compounds wherein the catalyst is more readily separated from the reaction mixture. It would further be desirable to develop a process for the preparation of phenolphthalein compounds that reduces waste generation, for example by allowing the catalyst to be reused. It would also be desirable if these processes provided phenolphthalein compounds in high purity.